Device for the infinitely variable correction of the marginal field of a magnet



Dec. 17, 1968 P. TSCHOPP 3,417,356

DEVICE FOR THE INFINITELY VARIABLE CORRECTION OF THE MARGINAL FIELD OF A MAGNET Filed Feb. 13, 1967 2 Sheets-Sheet 1 BY 6, J

ATTORNEYS Dec. 17, 1968 P TscHdPP 3,417,356

DEVICE FOR THE INF-I NITELY VARIABLE CORRECTION OF THE MARGINAL FIELD OF A MAGNET Filed Feb. 13. 1967 2 Sheets-Sheet z FIG. 7 F768 INVENTOR. PE 75! Tiff/OPP ATTORNEYS United States Patent 3,417,356 DEVICE FOR THE INFINITELY VARIABLE CORRECTION OF THE MARGINAL FIELD OF A MAGNET Peter Tschopp, Zurich, Switzerland, assiguor to Maschiuenfabrik Oerlikon, a corporation of Switzerland Filed Feb. 13, 1967, Ser. No. 615,785 Claims priority, application Switzerland, Feb. 15, 1966, 2,196/ 66 7 Claims. (Cl. 335-298) ABSTRACT OF THE DISCLOSURE Hollow magnetic elements are placed around the opposed poles of a magnet to permit correction of the field gradient at the edge of the magnetic field in the air gap between the poles. Elements are adjustable axially of the poles by sliding or screw thread engagement therewith. The longitudinal cross-sectional profile of the element may vary, and more than one element may be concentrically arranged on each pole. The element may be a cylinder or a rectangular frame. The frame may comprise several parts, some of which may be non-magnetic.

The present invention relates to a device for the infinitely variable correction of the edge field between the poles of a magnet, with the use of magnetic correction elements.

The magnetic field in the air gap between two parallel pole surfaces of a magnet can be considered completely homogeneous in the case of a large area of the pole surfaces as compared with the length of the air gap. However, in the case of a pair of poles with a relatively small surface area as compared with the length of the air gap, a radial field gradient is present in the edge zone of the magnetic field, which gradient makes itself perceptible in a corresponding, generally undesired, non-homogeneity of the magnetic field in the air gap. Apart from the geometry of the air gap, i.e., the diameter and length of air gap and the ratio between them, the deviations also depend on the intensity of the magnetic field. Such non-homogeneity, therefore, requires correction in order to make the field homogeneous. On the other hand, for certain purposes a certain non-homogeneous field pattern may be desired. This can also be obtained by correction.

To correct the edge field, it is known to apply magnetic correction bodies, also known as shims, to the poles. In one known embodiment, the poles have an annular shoulder surface carrying a magnetic ring fastened by screws to the pole surfaces, an annular film of desired thickness being interposed between the ring and pole surface. The number and thickness of the films determine the position of the ring, i.e., the protrusion thereof with respect to the pole surfaces, and therefore the amount of the correction of the edge field. For a precise correction arrangement, a larger number of very thin films is necessary. Since every different magnetic field intensity requires a different correction, frequent replacement of the films is necessary. Replacement can be effected only after loosening the screws and removing the rings, which is difiicult in view of the limited accessibility in the air gap. Frequently an optimum correction of the edge field is possible only experimentally, i.e., by the evaluation of corresponding measurements of the field strength in the critical edge zone. The optimum adjustment of the ring can therefore be cumbersome and time consuming. Furthermore the screw holes in the poles cause deviations in the field.

In accordance with the present invention, a particularly 3,417,356 Patented Dec. 17, 1968 simple arrangement for correcting the edge field, which does not have these defects, includes on each pole, at least one elongated correction element in the form of a magnetic hollow body which is arranged directly on the pole and around its periphery. The correction element can be adjusted with infinite variability on the pole in the direction of the field, and locked in position. One advantage of the invent-ion consists in the fact that the correction of the edge field is eifected in an infinitely variable manner Without interruption of the field measurements. Furthermore, no changes at the poles, such as screw holes for fastening bolts, are necessary. Instead, the correction bodies can be fastened by simple known means to the periphery of the pole.

The invention will be described by way of example, in greater detail below, with reference to the drawings, in

which identical parts are provided with the same reference numbers.

In the drawings:

FIG. 1 is an electromagnet provided with correction elements in accordance with the invention;

FIG. 2 shows a correction element with fastening means;

FIGS. 3 and 4 show correction elements having difierent shapes;

FIG. 5 shows a correction element accommodated within the magnet winding;

FIG. 6 shows concentrically arranged correction elements;

FIGS. 7 and 9 show correction elements screwed to the poles;

FIG. 8 shows an adjusting device associated with a correction element; and

FIGS. 10 and 11 show correction elements of rectangular shape.

The electromagnet 1 of FIG. 1, including an iron core 2 carrying windings 3 and 4, has an air gap 7 between its poles 5 and 6. The poles 5 and 6 are of circular crosssection, and support on their peripheries correction elements in the form of hollow cylinders 8 and 9, respectively, of magnetic material. The hollow cylinders are movable over the poles in the direction of the field so that the protrusion of the cylinder end surfaces 10 (FIG. 2) beyond the pole surface 11 can be adjusted as desired. The length of the hollow cylinder is so selected that in the entire range of adjustment the contact surface between the pole periphery and the hollow cylinder is sufiiciently large to pass the magnetic flux. To correct the edge field in the air gap, the hollow cylinders are displaced in the direction of the field, while simultaneously measuring the intensity of the field in the air gap until the desired path of the field has been reached. By means of a set screw 13, which presses via a pressure plate 14 against the pole, the hollow cylinders are locked in the adjusted position. The movement of the cylinders can be effected by hand or by means of an adjusting mechanism.

Instead of a rectangular cross-section, the hollow cylinder can have any desired longitudinal cross-sectional profile, depending upon the nature and amount of the required correction. FIG. 3 shows a hollow cylinder having a continuously extending profile 15, and FIG. 4 a hollow cylinder having a stepped profile 16. The profile shape can extend to the periphery of the hollow cylinder.

The poles frequently extend only slightly out of the windings, in which case there is insufiicient room for a hollow cylinder of larger length. In such a case, the 'winding 17 (FIG. 5) can be formed in part with a larger inside diameter so that the hollow cylinder 8 can be received in the annular space 18.

In order to effect a more precise correction of the edge field, a plurality of hollow cylinders can be arranged on the periphery of the pole. FIG. 6 shows an arrangement of two concentric hollow cylinders 8 and .19. Both hollow cylinders are adjustable independently of each other in the direction of the field. The hollow cylinder '8, as already described is supported on the periphery of the pole, while the hollow cylinder 19 is supported on the hollow cylinder -8.

To permit fine adjustment of the hollow cylinders, the poles and the hollow cylinders can be provided with a thread. FIG. 7 shows a pole 20 with an external thread 21 and a hollow cylinder 22 with a cooperable inner thread 23. The hollow cylinder 22 is brought to the correct position of adjustment by rotating it. This can be done by hand or by means of an adjusting mechanism such as shown in FIG. 8. The hollow cylinder 24 of F'IG. 8 is provided on its periphery with a toothed rim 25 with which a worm 26 can be brought into engagement. The worm 26 is fastened on a shaft 27 which is rotatably supported outside the air gap, and provided with a measuring device (not shown) for fine adjustment.

The hollow cylinder 28 of FIG. 9 is provided with an inner thread 29 extending only partially along its length. A smooth part 30 of the inner surface extends to the end edge of the hollow cylinder. In this way, the possibility of the inner thread influencing the field is avoided. The thread 31 of the pole 32 is cut on the thickened part 33 of the pole.

If the poles are of rectangular cross-section, the correction element can be a rectangular magnetic frame 34, as shown in FIG. For the adjustment, locking in position, and profiling of such a frame, the same applies as stated above with regard to a cylindrical correction element. Instead of being made of one piece, the frame can consist of two or more pieces, as may be desired, for convenience of manufacture in the case of large rectangular poles. FIG. 11 shows a frame 35 which consists of the parts 36, 37, 3-8 and 39. Each part can be adjusted on the periphery of the pole independent of, or together with, the other parts and then fixed in position, for instance, by means of set screws 40 and 41 as shown in connection with the part 37. In certain situations, it is necessary to correct the edge field only on two opposite sides of the air gap. In such a case, only the parts on the sides in question consist of magnetic material, for instance, the parts 36 and 37. These parts can then be connected by parts 38 and 39 of non-magnetic material.

What is claimed is:

1. An arrangement for the infinitely variable correction of the edge field in the air gap between two opposed poles of a magnet, comprising a hollow magnetic correction element surrounding the periphery of each pole, the internal diameter of the portion of said correction element engaging the pole being substantially equal to the largest external diameter of the pole, so that no gap exists between the end ofthe correction element and any portion of the pole, said correction element being adjustable with respect to the pole in the direction of the field in an infinitely variable manner, and means for locking said correction element to the pole in any desired position of adjustment.

2. An arrangement as defined in claim 1 including a winding surrounding each magnetic pole, at least part of the internal diameter of said winding being larger than the external diameter of said pole, and said element being accommodated at least in part within said winding.

3. An arrangement as defined in claim 1 including a plurality of correction elements arranged concentrically with respect to each other.

4. An arrangement as defined in claim 1 wherein said correction elements are hollow cylinders.

5. An arrangement as defined in claim 1 wherein each of said correction elements is a hollow rectangular frame.

6. An arrangement as defined in claim 5 wherein the sides of said frame are separate parts.

7. An arrangement as defined in claim 6 wherein some of said parts are made of non-magnetic material.

References Cited UNITED STATES PATENTS 3,134,933 5/1964 Brand et al. 335-281 3,018,422 1/1962 Seaton 335298 3,253,194 5/1966 Parker 335302 3,325,758 6/1967 Cook 335217 GEORGE HARRIS, Primary Examiner.

US. Cl. X.R. 

